This invention relates to a new and improved electron gun evaporation source which uses high direct current negative voltage (HV) to form high energy electron beams for high vacuum evaporation for producing thin-film coatings on various substrates. More particularly, the invention relates to the use of a grounded metallic shield which functions as an electrode enclosing the filament leads and emitter structures of an electron beam source in a high vacuum chamber and a gas bleed to protect the filament of the gun.
High vacuum electron beam heated evaporation (and sublimation) sources are used in coaters for the manufacture of thin-film optical devices, semiconductor devices used in integrated circuits and many other devices employing thin-film technology. The voltages used in these applications vary between about 4 kV and up to 30 kV with a nominal value of 10 kV.
It has been found that in the operation of electron beam sources, the operation can be disrupted by the build up of a glow discharge or worse, the initiation of an arc. This has an adverse effect that besides interrupting the application of power for evaporation, an arc can be struck which can melt and destroy vital parts of the coater. The solution to this problem has been the use of power supplies which can detect the rapid increase in current which signals the start of an arc. The use of very rapid switching devices often based on tetrode tubes then reduces the voltage (and thus power) to zero. The use of these rapid xe2x80x9cswitch-offxe2x80x9d power supplies is perfectly acceptable when evaporation is performed in high vacuum better than 10xe2x88x924 Torr.
When thin-films of materials used in optical coatings are required, it has been found that particularly when oxides are used, it is necessary to add oxygen to the coating system to assure that the coating is not deficient in oxygen but remains stoichiometric. This is a result of the mode of evaporation of compounds which often evaporate as their constituents and only recombine on the surface. As an example, silica evaporates (or more correctly sublimes) as SiO and O2. It is because the oxygen is more mobile and more rapidly pumped away that additional gas must be added to the coater so that adequate oxygen appears at the condensing surface and SiO2 is formed instead of SiOx where x less than 2. Consequently, many optical coatings have to be conducted like reactive evaporations and the electron beam source must operate in the pressure range between 10xe2x88x924 and 10xe2x88x923 Torr.
In coaters using high power electron beam evaporation at these xe2x80x9chighxe2x80x9d pressures, without special precautions, the rapid occurrence of arcs cannot be controlled by the rapid xe2x80x9cswitch-offxe2x80x9d power supplies, and the production of the thin-film is effectively prevented.
This effect was first described in 1889 by F. Paschen who found that the voltage breakdown could be related to the product of the pressure and distance of the gap between the electrodes. This effect was used in the design of high voltage vacuum equipment by L. Holland and was described in his book xe2x80x9cVacuum Deposition of Thin Filmsxe2x80x9d published in 1966. On page 91 Holland says, xe2x80x9cThus undesirable discharges can be prevented by bringing the anode electrode near the cathode surface . . . xe2x80x9d and a drawing is shown in which a high voltage lead is enclosed in a grounded shield having holes in it to assist pump down. For a particular pressure condition, the corresponding gap is called the xe2x80x98dark spacexe2x80x99. This is defined more particularly as it relates to thin film engineering in a book by D. Mattox published in 1998 entitled xe2x80x9cGlossary of terms and acronyms used in surface engineeringxe2x80x9d. The xe2x80x9cCathode Dark Spacexe2x80x9d is defined there as xe2x80x9cThe darker region of a plasma near the cathode surface where most of the potential drop in a DC diode discharge occurs. Region where electrons are being accelerated away from the cathode. Also called the Cathode sheathxe2x80x9d. The xe2x80x9cDark Space Shieldxe2x80x9d is defined there as xe2x80x9cA grounded surface that is placed at less than a dark space width from the cathode in order to prevent establishing a discharge in the region between the two surfaces. Also called the Ground shield. See Paschen curve.xe2x80x9d The use of the xe2x80x98dark spacexe2x80x99 has been used for many years to insulate components of differing voltage in vacuum environments.
While it is known how to reduce arcs even at xe2x80x98highxe2x80x99 pressure of 10xe2x88x923 Torr, the life of the electron beam emitter filament is substantially reduced especially for the cases where oxygen is purposely introduced to the coater. This is because of the reaction first observed in incandescent filament lamps, that tungsten forms a volatile compound with oxygen which rapidly destroys the integrity of the filament.
The present invention reduces arcs at xe2x80x9chighxe2x80x9d operating pressures used in reactive and oxide evaporation and provides a condition where the filament emitter does not degrade by reaction with oxygen.
In accordance with this invention, it has been found that it is possible to make a filament from a metal which does not oxidize. Such a metal namely, iridium, a precious metal, is available. The work function of iridium is about 5.3 eV and is higher than tungsten which is about 4.5 eV. Furthermore, iridium has a substantially lower melting point (about 2454 deg C.) than tungsten (3410 deg C.). The Fredericks Company has solved this problem by coating the iridium with yttria which reduces the effective work function to 2.4 eV. We have used filaments like this and found that they are good substitutes for tungsten and do not oxidize but the cost of iridium is often prohibitive for commercial applications.
It has been found that if the shielded high voltage lead and gun structure is used together with a thin septum of metal such as stainless steel sheet about 0.010 inch thick placed over the exit place of the electron beam, an inert gas such as argon can be introduced to the shielded structure. When the electron beam is turned on initially, it automatically melts a hole in the stainless steel septum just big enough to allow it to escape. This technique was used by Chambers and Carmichael in their development of electron beam ion plating and was described in their paper in xe2x80x98Research and Developmentxe2x80x99 in May 1971 entitled xe2x80x9cProcessing parameters, measured with apparatus using electron beam evaporation that extends ion plating to a wider range of materials, provide a new basis for evaluating this third method of vacuum coatingxe2x80x9d.
In addition it has been found that at the vacuum levels of interest (10xe2x88x922 to 10xe2x88x924 Torr) if gas such as argon is introduced to the inside of the shield, the flow through the small hole in the septum or shield will be sonic. Consequently, if the gas introduced is inert the tungsten filament will be effectively shielded from any oxygen.